How to Test Car Engine Pistons

A compression test can reveal what's happening inside a car engine without tearing it apart. The cylinder walls, valves, seals, pistons and rings all create a tight seal, allowing energy to push the piston down into the cylinder. When these parts wear out, the engine doesn't function properly.

Difficulty:

Moderately Easy

Instructions

Things You'll Need

• Compression tester
• Motor oil

1. Prepare for the test by warming up the engine. The battery must be in good condition since it will spin the engine quite a few times. Testing a cold engine will give inaccurate readings.

2. Stop the engine and remove the spark plugs so that the starter can spin the engine freely, and then disable the ignition system. To do this, either pull the coil wire or disable the ignition coil.

3. Insert the compression tester into one of the spark plug holes and crank the starter to rotate the engine. This builds compression in that cylinder. Move on to the next cylinder by removing the compression tester and inserting it into the next spark plug hole, repeating this procedure until you test all the cylinders.

4. Record the compression number for each cylinder and compare them. The numbers should be about 10 PSI of each other and match up with manufacturer's recommendations. If one or more of the cylinders shows a difference of 15 or more PSI, a problem exists.

5. Remove the compression tester and add a small amount of motor oil to the cylinder that showed a low reading. Run the compression test again to test the piston rings and cylinder walls for wear. If the second compression number is higher than the first, the problem lies here. If it stays the same or is very low, the problem is more than likely due to worn valves or serious internal damage.